One of the criteria for imaging systems is the amount of throughput that can be obtained; that is, the number of patients that can be processed and from which complete data can be obtained in a given amount of time. Anything that can be done to increase throughput, that is decrease the time required to obtain data from the patient is a plus as long as the data that is acquired is useful and as long as there is not increased risk of any kind, or increased discomfort to the patient during the scan.
Presently, separate images of two different spectral components such as water and lipids within the patient are sometimes obtained. The separate images are important for diagnostic purposes; since they supply the user with chemical information in addition to the morphological or anatomical information of conventional imaging. Moreover by using an appropriate shift of one image with respect to the other the two images can be combined resulting in an image free of chemical shift artifacts. However, at the present time to obtain the separate images at least two scans of the patient are required; i.e., two imaging cycles such as spin echo cycles have to be processed to obtain the two images.
A unique pair of interelated sequences to obtain information on water and/or lipids in a patient was described in an article appearing in Radiology, entitled "Simple Proton Spectroscopic Imaging" by W. T. Dixon (Vol. 153, pp. 189-194). In that article a method for encoding spectroscopic information into clinical images is explained. The image produced differentiates between the water and fat intensities. The method requires using a normal spin echo sequence in which the Hahn and gradient echoes coincide. In addition each excitation is repeated with the Hahn echo shifted by an appropriate interval. The 180 degree Rf pulse is shifted by a time T to shift the Hahn echo with respect to the gradient echo an amount 2T. The time T is sufficient to cause the chemical shift between the echoes of water and lipids to be 180 degrees out of phase at the gradient echo time. The image produced with the described sequence clearly indicates differences between the signals due to water and the signals due to fat.
By obtaining normal spin echo derived image data in addition to obtaining the modified spin echo image data, the two images can be constructed. Thus, the described method enables imaging two spectral components in a single image corrected for the chemical shift artifact or obtaining separate images of each of two spectral components.
A disadvantage of the described method is the amount of time required for obtaining the data for imaging. More particularly, two scans are required to obtain the two images. Any reduction in this amount of time required to obtain the two images would be advantageous and a sought after goal.
Accordingly it is an object of the present invention to obtain separate data contributions from first and second spectral components sufficient to construct an image for each of the components with a single magnetic resonance scan; thus, cutting the scan time of Dixon by at least one-half.
As used herein scan time is the time required to apply all of the excitation pulses and gradient pulses to enable acquiring sufficient data to construct an image of a selected volume of a sample being imaged. A single scan is the minimal Rf signal transmitting and receiving repetitions required to acquire the data for an image of a single spectral component having the desired spatial resolution and signal-to-noise ratio.